sharpd: Fix memory leaks related to SRv6 nexthops

sharpd: Fix memory leaks related to SRv6 nexthops

Running the `zebra_seg6local_route` topotest with `--valgrind-memleaks`
gives several memory leak errors. This is due to the way SRv6 routes
(seg6 and seg6local routes) are handled: when the user executes the CLI
command `sharp install seg6-routes` or `sharp install seg6local-routes`
to create a seg6 or seg6local route, sharpd calls
`nexthop_add_srv6_seg6` or `nexthop_add_srv6_seg6local` to create an
SRv6 nexthop. A pointer to the SRv6 nexthop is stored in the global data
structure `sg.r.nhop`. If you call `sharp install routes`,
`sharp install seg6-routes` or `sharp install seg6local-routes` to create
more routes, `sg.r.nhop` is set to zero and the
pointer to the SRv6 nexthop contained in `sg.r.nhop` is definitely lost
and the allocated memory is never freed.

This patch adds calls to `nexthop_del_srv6_seg6()` and
`nexthop_del_srv6_seg6local()` to free the memory allocated for the SRv6
nexthop before clearing the `sg.r.nhop` data structure.

Signed-off-by: Carmine Scarpitta <carmine.scarpitta@uniroma2.it>